The invention relates to a suction roll of a paper machine comprising a perforated shell and a static suction box sealed against the inner surface of the shell to create a suction zone on the surface of the suction roll while the shell rotates.
Suction rolls provided with a suction box are used particularly in the forming section and the press section of paper and board machines at locations where a high local vacuum is needed. The suction box sealed against the inner surface of the roll shell is connected to a vacuum system, whereby a local vacuum of desired magnitude and, as a result of it, a flow of water or air into the suction roll are produced in that sector of the shell which is at the suction box each time when the roll is rotating. The holes drilled through the shell are generally provided with countersinks, or counterbores, in the outer surface of the shell, the function of which countersinks is to reduce the size of unbroken connecting portions in the outer surface of the shell and to enlarge the open area of the outer surface. In the suction rolls of prior art, the countersink surrounds the suction hole symmetrically. Both the countersink pattern and the bore pattern are generally regular, which is important in order to provide an even suction effect and to prevent marking of the web.
One considerable problem associated with suction rolls is the noise they generate. At the end of the suction zone, a vacuum prevails in the individual holes of the shell, which vacuum is filled with air suddenly and noisily after the seal of the trailing side. As the roll comprises thousands of holes and the phenomenon is repeated in rapid succession, the ear distinguishes only a continuous tune, whose pitch depends on the speed of the roll and on the length of the regular drill pattern. Noise peaks can additionally occur at certain multiples of rotation frequency. If the holes are in axial rows, an entire row produces simultaneously a sound impulse, with the result that the noise which is produced may become intolerable at high speeds. By arranging the drill pattern of the holes to be spiral-like, the number of the holes filled simultaneously can be made smaller at the same time as their spacing is reduced, whereby the volume of sound decreases. However, in many cases this is not sufficient to attenuate the noise to an acceptable level.
The aim of the invention is to reduce the noise caused by suction rolls.